1. Field of the Invention
The invention relates to a method for the surface treatment of inorganic pigment particles, particularly titanium dioxide pigment particles, and use of the pigment particles in laminates, coatings and plastics.
2. Description of Related Art
Inorganic particles, particularly inorganic pigment particles, are often surface-treated in order to modify certain properties, such as surface charge, dispersing properties, acid resistance or light-fastness. When using pigments, particularly titanium dioxide, it is important, among other things, to achieve high hiding power (opacity), which can be improved by a special type of surface treatment. High hiding power or opacity is required when using the titanium dioxide pigment particles in emulsion paints or laminates (decorative laminating paper), for example.
Elevated hiding power is customarily obtained by surface treatment with silicon oxide and/or aluminum oxide, which has a structure that is as fluffy, porous and voluminous as possible and acts as a spacer for the individual pigment particles. Alternatively, the pigment particles can be mixed with suitable extenders (e.g. calcium carbonate, kaolin, talcum), in which case the extender particles are likewise intended to serve as spacers for the pigment particles.
U.S. Pat. No. 3,591,398 and U.S. Pat. No. 4,075,031 disclose methods for precipitating porous SiO2 and Al2O3 coatings onto TiO2 particles, in order to improve the hiding power of the pigment particles. In this context, alkaline precursors of the coating oxides are added to an acid, aqueous suspension of the TiO2 particles displaying a pH value of ≦7, and the corresponding oxides are rapidly precipitated out in a fluffy structure.
Decorative laminating paper forms part of a decorative, thermosetting coating material, used with preference for finishing furniture surfaces and for laminate flooring. Laminates is the term used to denote materials in which, for example, several impregnated, stacked layers of paper, or paper and hardboard or particle board, are pressed together. The use of special synthetic resins achieves extraordinarily high resistance of the laminates to marring, impact, chemicals and heat.
The use of special-purpose papers (decorative laminating papers) permits the production of decorative surfaces, where the decorative laminating paper serves not only as facing paper, e.g. to hide unattractive wood material surfaces, but also as a carrier for the synthetic resin.
The demands imposed on decorative laminating paper include, among others, opacity (hiding power), light-fastness (greying resistance), color-fastness, wet strength, suitability for impregnation and printability. The cost-effectiveness of the decorative laminating paper production process is determined by the opacity of the pigment in the paper, among other things.
In principle, a pigment based on titanium dioxide is eminently suitable for achieving the necessary opacity of the decorative laminating paper. During paper manufacture, a titanium dioxide pigment, or a titanium dioxide pigment suspension, is usually mixed with a pulp suspension. In addition to pigment and pulp as the feedstock, use is generally also made of auxiliaries, such as wet-strength agents, and further additives, such as certain fillers, where appropriate.
To improve the light-fastness (greying resistance) of decorative laminating paper, the titanium dioxide pigment is customarily coated with aluminum compounds, particularly with aluminum phosphate. Special surface treatments for improving the opacity of the titanium dioxide pigments when used in decorative laminating paper and other applications are likewise known, such as the precipitation of high-volume metal oxide layers on pigments for emulsion paints, where the precipitate acts as a spacer for the individual pigment particles.
According to DE 103 32 650 A1, it is possible to manufacture a titanium dioxide pigment with high greying resistance that simultaneously displays improved retention and opacity. The method is characterized in that an aluminum component and a phosphorus component are added to a titanium dioxide suspension at a constantly maintained pH value of at least 10, after which the pH value is reduced to below 9 in order to precipitate aluminum phosphate compounds.
DE 10 2011 015 856 A1 describes a further method for manufacturing an aluminum phosphate-coated titanium dioxide pigment with high greying resistance and good brightness and opacity. In this method, phosphoric acid is first added to the titanium dioxide suspension, followed by an alkaline and an acid aluminum compound.
WO 2002/077107 A2 discloses a titanium dioxide pigment composition displaying both high greying resistance and high opacity. The pigment composition consists of a blend of two different titanium dioxide pigment types. Type A is a titanium dioxide pigment with high greying resistance suitable for use in decorative laminating paper, e.g. coated with aluminum phosphate, and Type B is a titanium dioxide pigment suitable for use in emulsion paints that is characterized by coating with an elevated SiO2 and Al2O3 content precipitated in flaky, high-volume form. This composition has the disadvantage that two different pigment types first have to be produced and subsequently blended homogeneously.
There is a need for a method for manufacturing inorganic pigments that display advantages compared to known inorganic pigments, e.g. in the form of higher efficiency and cost savings that can be achieved in use as a result. There is especially a need for an alternative, economical method for manufacturing a titanium dioxide pigment with improved opacity and the possibility of simultaneously also optimizing other properties, such as weather resistance, greying resistance, brightness, corrosion resistance, etc.